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Results and Discussion: During the prone bridge the addition of an exercise ball resulted in increased myoelectric activity in the rectus abdominis and external oblique.. The addition of

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Trunk muscle activity during bridging exercises on and off a

Swissball

Gregory J Lehman*, Wajid Hoda and Steven Oliver

Address: Department of Graduate Studies, Canadian Memorial Chiropractic College, Toronto, ON, Canada

Email: Gregory J Lehman* - glehman@cmcc.ca; Wajid Hoda - whoda@cmcc.ca; Steven Oliver - soliver@cmcc.ca

* Corresponding author

EMGtrunk stabilityexerciseswiss ballrehabilitation

Abstract

Background: A Swiss ball is often incorporated into trunk strengthening programs for injury

rehabilitation and performance conditioning It is often assumed that the use of a Swiss ball

increases trunk muscle activity The aim of this study was to determine whether the addition of a

Swiss ball to trunk bridging exercises influences trunk muscle activity

Methods: Surface electrodes recorded the myoelectric activity of trunk muscles during bridging

exercises Bridging exercises were performed on the floor as well as on a labile surface (Swiss ball)

Results and Discussion: During the prone bridge the addition of an exercise ball resulted in

increased myoelectric activity in the rectus abdominis and external oblique The internal oblique

and erector spinae were not influenced The addition of a swiss ball during supine bridging did not

influence trunk muscle activity for any muscles studied

Conclusion: The addition of a Swiss ball is capable of influencing trunk muscle activity in the rectus

abdominis and external oblique musculature during prone bridge exercises Modifying common

bridging exercises can influence the amount of trunk muscle activity, suggesting that exercise

routines can be designed to maximize or minimize trunk muscle exertion depending on the needs

of the exercise population

Background

Trunk muscle co-activation of several muscles is

consid-ered necessary in achieving adequate spinal stability to

prevent and treat low back injury [1] Common exercise

recommendations from health professionals include

trunk exercises to prevent and treat low back injuries

Knowing the trunk muscle activation levels during

cises is important in the prescription and design of

exer-cise programs that aim to increase the training intensity

over time (progressive resistance model) Previous

research has documented trunk muscle EMG during vari-ous exercises designed to train the trunk musculature and during functional activities [2-7] Ng et al [7] found that abdominal and trunk muscles not only produce torque but also maintain spinal posture and stability during axial rotation exertions Vera-Garcia et al [8] showed that per-forming curl-ups on a labile (moveable) surface changes the muscle activity amplitude required to perform the movement Increases were greatest in the external oblique muscles Mori [9] documented the trunk muscle activity

Published: 30 July 2005

Chiropractic & Osteopathy 2005, 13:14 doi:10.1186/1746-1340-13-14

Received: 28 April 2005 Accepted: 30 July 2005

This article is available from: http://www.chiroandosteo.com/content/13/1/14

© 2005 Lehman et al; licensee BioMed Central Ltd

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Chiropractic & Osteopathy 2005, 13:14 http://www.chiroandosteo.com/content/13/1/14

during a variety of trunk muscle exercises on a Swiss ball

However, comparisons in muscle activity were not made

with ground based exercises (no Swiss ball present),

there-fore, the influence of a Swiss ball on trunk muscle activity

compared with ground based bridging is not known

The importance of trunk muscles in providing adequate

spine stability is well established and the role of trunk

muscles during a variety of tasks has been well

docu-mented Swiss balls are a common addition to trunk

mus-cle exercises In fitness centres and rehabilitation centres,

Swiss balls are often touted as being superior to ground

based exercises in their ability to recruit trunk muscles

(rectus abdominis, external oblique, internal oblique,

erector spinae) Considering the ubiquity of Swiss balls,

one research question was posed: Does the addition of a

Swiss ball to bridging exercises influence trunk muscle

activity?

The implications of this study are twofold: 1 Modifying

trunk muscle activity could be important in the safety and

efficacy of rehabilitation exercises when a low level of

trunk muscle activity is desired; and 2 Identifying

exer-cises which maximally activate the trunk muscles may

make it possible to develop an efficient and less time

con-suming general strength program that conditions the

trunk muscles

Methods

Patient Characteristics and Inclusion Criteria

An all male study population (n = 11, average weight =

85.4 kg (13.1), average height = 179 cm (11) and age 27.6

(3.2) with greater than six months of weight training

expe-rience, without back pain or upper limb injuries, was

recruited from a convenience sample of College students

Each subject signed an information and consent form,

approved by the Research Ethics Board (Canadian

Memo-rial Chiropractic College) explaining the procedures and

risks involved with study participation

Protocol Overview

The subjects performed five different trunk muscle

exer-cises on two different surfaces (stability ball and floor)

and two separate normalization tasks

EMG Data Collection Hardware Characteristics

Disposable bipolar Ag-AgCl disc surface electrodes with a

diameter of 1.0 cm were adhered bilaterally over the

mus-cle groups studied with a centre to centre spacing of 1.5

cm EMG electrodes were placed parallel with the muscle

fibres on the skin above the rectus abdominus, external

oblique, internal oblique and lower erector spinae (L3)

on the right side of each subject The raw EMG was

ampli-fied between 1000 and 20,000 times depending on the

subject The amplifier had a CMRR of 10,000:1 (Bortec

EMG, Calgary AB, Canada) Raw EMG was banned pass filtered (10 and 1000 Hz) and A/D converted at 2000 Hz using a National Instruments data acquisition system

EMG Normalization Procedure

In order to compare values of muscle activity across sub-jects it was necessary to normalize the EMG data This required that all EMG values be expressed as a percentage

of the maximum EMG activity that can be produced vol-untarily by a muscle Subjects performed two repetitions

of two different maximal voluntary contractions (MVC) The subjects were first required to perform a three second maximal isometric trunk curl up and twist against an immovable resistance to maximally recruit the rectus abdominis, internal oblique and external oblique mus-cles Second, the subjects performed an isometric prone trunk extension against a fixed resistance to recruit the erector spine and multifidus musculature The muscle activity during all subsequent experimental tasks was expressed as a percent of the peak activity found during the normalization procedure (MVC exercises) Subjects were allowed to familiarize themselves with the move-ments before muscle activity was recorded

Description of Exercise Tasks

Feedback from instructors was given in order to achieve a consistent spine and lower limb posture during the fol-lowing tasks Subjects aimed to keep their spines in a neu-tral position with their legs parallel to their trunk during the bridging exercises The following tasks were chosen because they are common exercises performed in rehabil-itation and exercise programs No attempt was made to control for the different body position relative to gravity between the different exercises It is recognized that the body's position relative to gravity and the influence of gravity is different between exercises using a Swiss ball and those on the ground Therefore, conclusions regarding the influence of an unstable surface on trunk muscle cannot

be made as the body's position confounds this The side bridge was added to give the reader a frame of reference for the muscle activity found during the other exercises It was not performed on the Swiss ball as this exercise is not commonly performed on a Swiss ball and the participants were not familiar with the exercise Figures 1, 2, 3, 4, 5 illustrate the exercises investigated Two trials of each of these tasks were recorded EMG data was collected for 5 seconds during the isometric portion each task The tasks the participants were required to complete were as follows:

1 Supine Bridge – Subjects began by lying supine on the floor with their feet flat on ground, knees bent 90 degrees, toes facing forward and hands on the floor by their sides, palms facing down Pushing through the heels, subjects lifted their pelvis off the ground to form a plank

Supine bridge

Figure 1

Supine bridge

Supine bridge on swiss ball

Figure 2

Supine bridge on swiss ball

Chiropractic & Osteopathy 2005, 13:14 http://www.chiroandosteo.com/content/13/1/14

Prone Bridge

Figure 3

Prone Bridge

Prone bridge on Swiss ball

Figure 4

Prone bridge on Swiss ball

2 Supine Bridge with Stability Ball – The same procedure

was applied as in task #1, however, in this variation the

individuals placed their feet flat on a stability ball

3 Prone Bridge – Subjects assumed a prone position on

the floor, and when instructed established a prone plank

position with elbows placed beneath the shoulders and

upper arms perpendicular to the floor In this position

only the feet and the forearms were touching the floor

4 Prone Bridge with Stability Ball – The same procedure

was applied as in task #3, however, in this variation the

individual's forearms were placed on a stability ball

5 Side Bridge – Subjects assume a side plank position

with elbow under shoulder and upper arm perpendicular

to the ground

EMG Processing

The normalization tasks and the exercise tasks for both

studies were processed in an identical manner Raw EMG

from each trial was smoothed using an RMS averaging

(window of 100 ms, 50 ms overlap) technique The

aver-age activity, expressed as a percent of the normalization

contraction, was found for the exertion portion of each

exercise and repetition The average of two repetitions for

each exercise and for each muscle was then calculated

Statistical Analysis

A repeated measures ANOVA with a post hoc Tukey test was used to determine activation level differences within each muscle across bridging exercise tasks

All statistical tests were performed at the 5% level of significance

Results

Table 1 depicts the muscle activation levels across exer-cises The addition of an exercise ball did not influence the muscle activity in the Internal Oblique (Figure 6) in both bridging exercises During the prone bridge the addition

of an exercise ball resulted in increased myoelectric activ-ity in the rectus abdominis and external oblique (Figure 7 and Figure 8) The exercise ball did not influence the Rec-tus Abominis or the External Oblique muscle activity dur-ing a supine bridge The addition of an exercise ball did not influence the Erector Spinae (Figure 9) activity during the supine bridge or the prone bridge

The side bridge produced the highest myoelectric activity

in both the Internal Oblique and Erector Spinae The prone bridge with arms on a Swiss ball produced the high-est myoelectric activity in both the Rectus Abdominis and External Oblique

Side bridge

Figure 5

Side bridge

Chiropractic & Osteopathy 2005, 13:14 http://www.chiroandosteo.com/content/13/1/14

Discussion

The primary aim of this study was to determine if

per-forming bridging exercises on a Swiss ball rather than the

ground resulted in increases in trunk muscle activity A

blanket statement that a more labile surface (the Swiss

ball) increases trunk muscle activity cannot be made The

influence of surface stability on muscle activity appears to

be muscle and exercise dependent For example, during

the prone bridge the primary mover (the rectus abdominis

resisting trunk extension) was the most influenced by the addition of a Swiss ball Conversely, during a supine bridge, one of the primary movers, the erector spinae, was not influenced by surface stability (the other primary mover, the Gluteus Maximus was not studied) It may be argued that the increase in activation levels of the external oblique and the rectus abominis during prone bridging appear to be caused by decreases in surface stability and not different biomechanical demands due to the body's

Table 1: Muscle activation levels expressed as percentage of the Maximum Voluntary Contraction for bridging exercises on different surfaces.

Exercise Pr Br Floor Pr Br Ball Side Bridge Su Br Ball Su Br Floor

Different From* 2,4,5 1,3,4,5 2,4,5 1,2,3 1,2,3

Different From* 2,4,5 1,3,4,5 2,4,5 1,2,3 1,2,3

Different From* 3,4,5 3,4,5 1,2 1,2 1,2

* This row indicates which other exercise the myoelectric signal for the respective column is statistically different (p < 05) from (Avg = Average muscle activity in % MVC, Stdev = Standard deviation, IO = Internal Oblique, RA = Rectus Abdominis, EO = External Oblique, ES = Erector Spinae

Pr Br = Prone Bridge, Su Br = Supine Bridge.

Internal oblique group average activity on/off a Swiss ball

dur-ing bridgdur-ing exercises

Figure 6

Internal oblique group average activity on/off a Swiss ball

dur-ing bridgdur-ing exercises

Rectus abdominis group average activity on/off a Swiss ball during bridging exercises

Figure 7

Rectus abdominis group average activity on/off a Swiss ball during bridging exercises

position relative to gravity This finding agrees with the

Vera-Garcia et al [8] study that investigated trunk curl up

exercises While there were differences in the body's

posi-tion relative to gravity between the ground exercise and

the ball exercise during prone bridging, performing the

bridge on a ball finds the participant in a more vertical

position This suggests there is less force creating a trunk

extension movement (i.e gravity attempts to increase

lor-dosis which is resisted by muscle activity) due to the fact

that the centre of mass of the trunk and head segment

would be closer to the axis for trunk extension Therefore

less muscle activity may have been generated to resist this torque (compared with the ground based bridge) and more muscle activity may have been required to produce secondary spinal stabilization due to the labile surface

An important observation from all exercise tasks was the large variability in muscle activity between subjects that can greatly influence the interpretation of these results Figure 10 illustrates an example of this variability Figure

6 shows the average activity in the internal oblique muscle during prone bridging on and off a Swiss ball for one rep-etition from each subject This indicates that some sub-jects showed large changes in muscle activity while others showed minimal changes when modifications to the exer-cise tasks were made It is possible that some subjects voli-tionally contracted their trunk muscles to provide stability while some others may have not It is possible that indi-viduals may be able to influence their trunk muscle activ-ity either through verbal encouragement, or feedback produced by electromyography Additionally, the varia-bility may have been due to slight variations in participant posture or task performance While exercise standardiza-tion was sought through verbal correcstandardiza-tion of form, it is possible that differences in task performance between the subjects still occurred Further research may wish to deter-mine the influence of electromyographic feedback on influencing the trunk activation levels during resistance exercise This may decrease the variability between subjects

This study is limited because it only measured the trunk muscle activity during the various exercises No measure-ments were made nor a biomechanical model constructed

External oblique group average activity on/off a Swiss ball

during bridging exercises

Figure 8

External oblique group average activity on/off a Swiss ball

during bridging exercises

Erector spinae group average activity on/off a Swiss ball

dur-ing bridgdur-ing exercises

Figure 9

Erector spinae group average activity on/off a Swiss ball

dur-ing bridgdur-ing exercises

Internal oblique muscle activity for each participant during one repetition of prone bridging on/off a Swiss ball

Figure 10

Internal oblique muscle activity for each participant during one repetition of prone bridging on/off a Swiss ball

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to determine the compressive or shear loading on the

spine during the tasks This type of kinematic, and

subsequently force data, is optimal when determining the

safety and tissue loading properties of various

move-ments Also, this study did not quantitatively measure

spi-nal posture This may influence the muscle activation

levels While a consistent spinal posture was encouraged

and monitored by the experimenters, it is also possible

that minor differences in spine posture did occur

Moni-toring spinal posture via a kinematic measurement system

(eg Electromagnetic tracking) may be important in future

work

While increased trunk muscle activation can result in

higher compressive loads on the spine [12], is this

amount of trunk muscle activity necessarily increasing the

risk of injury? We are unable to say if increases in activity

levels are due to biomechanical demands, or if they are

due to motor control decisions that permit enhancements

in spine stability that may decrease the risk of injury

Con-versely, if an exercise modification results in decreases in

muscle activity, is this always beneficial in terms of injury

prevention? Is it possible that subjects who lower their

muscle activation levels during tasks predispose

them-selves to a "buckling" type injury because sufficient spinal

stability is not created with the current amount of trunk

muscle activity [13]? It is important to note that this study

measured trunk muscle activity in an athletic young

homogenous population Sedentary individuals or those

with trunk or lower leg injuries may show different

results

Conclusion

Differences in trunk muscle activity are seen with the

addi-tion of a Swiss ball to bridging exercises It cannot be

con-cluded that these differences are solely due to changes in

surface stability due to the different biomechanical

demands of the exercises Future research should control

for exercise posture to determine how surface stability

influences muscle activity

Competing interests

There are no competing interests for this research project

Participants read and signed an information and consent

form approved by the Research Ethics Board (Canadian

Memorial Chiropractic College) The study protocol was

approved by the Research Ethics Board

Authors' contributions

GJL; study conception, study design, data collection,

sta-tistical analysis, manuscript preparation WH & SO: study

design, data collection, data processing

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